If I was going to label a fuse box, I'd do what I do when I label things. Use my Brother P-Touch. Easily one of the most useful purchases I have made in the past five years.
Of course it won't document a fusebox, but because I don't work with fuseboxes for a living and fuseboxes are not a hobby I am likely to take up, I can't see myself ever needing to do that enough times that finding a fusebox document would be easier than looking at a fusebox I already labeled.
